The present invention is directed to conductive coating compositions. More specifically, the present invention is directed to conductive coating compositions containing a diquaternary ammonium compound. The coating compositions of the present invention are suitable for preparing electrographic papers useful in electrographic imaging. One embodiment of the present invention is directed to a coating composition comprising a diquaternary ammonium compound of the formula ##STR2## wherein R.sub.1 is an alkyl group or alkylene group having from about 12 to about 22 carbon atoms, R.sub.2, R.sub.3, R.sub.4, R.sub.5, and R.sub.6 are independently selected from the group consisting of alkyl groups and aryl groups having from 1 to about 8 carbon atoms, n is a number from 1 to about 5, and A is an anion.
Conductive coating compositions, including those for electrographic papers, are known. In electrographic development processes, a charged latent image is generated directly on the electrographic paper. A toner carrying a charge opposite in polarity to that of the latent image is contacted with the paper and deposits on the charged area as a visible image. Further details regarding electrographic imaging processes are disclosed in, for example, U.S. Pat. Nos. 4,731,622; 4,485,982; 4,569,584; 3,611,419; 4,240,084; 3,564,556; 3,937,177; 3,729,123 and 3,859,960, the disclosures of each of which are totally incorporated herein by reference.
One example of a conductive coating composition is disclosed in U.S. Pat. No. 3,813,264, the disclosure of which is totally incorporated herein by reference. This patent discloses an electroconductive paper containing an N-(alkyl ammonium) acrylamide polymer. In addition, U.S. Pat. No. 3,835,102, the discloses of which is totally incorporated herein by reference, discloses a high polymer composition comprising a salt constituted from an integral type of polycationic polymer containing in its principal repeating unit quaternized nitrogen atoms and aromatic groups, an anion radical of a tetracyano compound, and a neutral tetracyano compound in appropriate amounts such that the electroconductivity is greater than 10.sup.-5 mho/cm. Further, U.S. Pat. No. 3,971,680, the disclosure of which is totally incorporated herein by reference, discloses an electroconductive paper comprising a substrate containing an effective amount of an electroconductive water soluble quaternary ammonium polymer. The polymer is prepared by reacting substantially stoichiometric amounts of at least one aromatic ditertiary amine and one or more anions containing organic compound.
Additionally, U.S. Pat. No. 3,663,461, the disclosure of which is totally incorporated herein by reference, discloses water soluble polyether polyelectrolyte salts containing quaternary nitrogen atoms in the polymer backbone and chain extended by ether groups. The polymers are prepared by treating the polymeric reaction product from an N,N,N',N'-tetraalkylhydroxy substituted diamine and an organic dihalide such as a dihaloalkane or a dihalo ether with an epoxyhaloalkane. Further, U.S. Pat. No. 3,642,663, the disclosure of which is totally incorporated herein by reference, discloses a polar solvent soluble electroconductive quaternized condensation product provided by reacting an alkylene polyamine, a polyalkylenepolyamine, or mixtures thereof with an epihalohydrin in a polar solvent to such an extent that there is little or no crosslinking in the condensation product and by quaternizing the condensation product with a quaternizing agent. In addition, U.S. Pat. No. 3,784,529, the disclosure of which is totally incorporated herein by reference, discloses polymeric quaternary compounds wherein the quaternized atoms are in the polymer chain backbone.
Electrographic papers typically are constructed with a base paper coated with layers of dielectric coating and conductive coating. The dielectric coating generally comprises a resistive resin binder, pigments, and additives, such as dispersing agents, lubricants, optical brighteners, plasticizers, and the like. The conductive coating typically contains conductive polymers, pigments, and additives, such as water-soluble resins, latices, dispersing agents, salts, humectants, optical brighteners, and the like. Generally, more than one coating is applied to the base paper. For example, a conductive coating is typically first coated on one surface of the base paper, followed by a dielectric coating on top of the conductive coating, and subsequently by another conductive coating on the other surface of the base paper.
The conductive polymers typically used in conductive paper coatings are generally water-soluble polyelectrolytes, which are hygroscopic and absorb considerable amounts of moisture. In the presence of water or moisture, the electrolyte polymers dissociate into ions carrying electrical charges and are electrically conductive. Because of the ionic characteristics of these materials, the conductivity of the polymer is a function of the moisture content, which, in turn, depends on the relative humidity conditions of the environment. At high relative humidity, the polymers are more conductive than they are at low relative humidity. Accordingly, with electroconductive papers coated with these polymers, a range or "window" of relative humidity values exists in which the electrographic process can best operate. This range typically is from about 35 percent relative humidity to about 65 percent relative humidity. At conditions lower than about 35 percent relative humidity, the conductivity of the conductive coating generally is inadequate, and the prints obtained generally exhibit low optical density and striation (vertical variations in print density in solid print areas).
In addition, papers coated with typically used conductive polymers tend to exhibit curling. Coatings containing polyelectrolyte conductive polymers tend to be dimensionally unstable as a result of their hygroscopic properties. Under high humidity conditions, the polymer absorbs more moisture and the coating tends to stretch. When the humidity is low, the polymer absorbs less moisture and the coating tends to contract. Unlike the conductive coating, the dielectric coating is relatively stable in dimension and does not stretch or contract under varying humidity conditions. The difference in humidity response between the conductive coating and the dielectric coating, therefore, contributes to the paper curling often observed with conventional electrographic papers.
A further shortcoming of papers coated with conventional polyelectrolyte conductive coatings is the high rigidity of the polymer at low relative humidity. The ability of an electrographic paper coated with these materials to conform itself to an electrographic writing head is negatively affected when the paper is too rigid. Because of poor flexibility of a rigid paper, the prints obtained often exhibit image breakup and line dropout.
In view of the shortcomings of conventional conductive coatings, a need exists for conductive coating compositions that result in minimal curling tendencies when coated onto paper. In addition, a need exists for conductive coating compositions for electrographic papers wherein the paper requires only two coatings, a dielectric coating on one surface and a conductive coating on the opposite surface. Further, there is a need for conductive coating compositions that enable electrographic papers suitable for use under a wide range of relative humidity conditions. Additionally, there is a need for conductive coating compositions that enable flexible electrographic papers which alleviate image breakup and line dropout, especially under low relative humidity conditions. There is a further need for electrographic papers suitable for use under low relative humidity conditions. In addition, a need exists for electrographic papers having a single conductive coating. A need also exists for electrographic papers that enable the generation of uniform, high density, and high quality electrographic images.